136
ALBERTO MONROY AND RACHELE MAGGIO
lasm of the egg and blastomeres. On the other hand there is no need to
emphasize how important it would be to know as much as possible about
the mechanisms and pathways of yolk resorption and of the utilization
of its breakdown products for the synthesis of embryonic proteins.
The appearance of the yolk platelets in the electron microscope
(Afzelius, 1956; our unpublished observations) shows that their main
body consists of tightly-packed tiny electron-dense granules. Each
platelet is surrounded by a thin granular membrane which is often lost
in the homogenates prepared with the usual sucrose procedure. Even
when this happens, the body of the platelet retains its appearance,
i.e., the elementary particles are not dispersed (unpublished observations). In some preparations of blastulae, the main body of some of
the platelets appeared to have patches of dense and clear zones.
Furthermore they were surrounded by a multi-layered membrane
similar to the one described by Karasaki (1959) in the tail-bud
stage of the amphibian embryo and interpreted by this author as an
indication of the decomposition of the main body that begins at its
surface. According to some preliminary observations (unpublished), the
platelets in this condition become lighter as they are sedimented at
higher centrifugal force. This may indeed be an expression of the progress
of their utilization. The significance of the lamellar structures surrounding the yolk platelets is at present obscure. No doubt it suggests the
possible transformation of the platelet into some other cytoplasmic
structure. Indeed, the possibility of the yolk being a source of new
mitochondria has been repeatedly suggested (Bellairs, 1958; Lanzavecchia and La Coultre, 1958; Sung, 1961) but no definite proof is yet
available. Histochemical tests have failed to indicate any change in the
yolk from the uncleaved egg to the pluteus. The general conclusion from
these observations has been that the main components of yolk are
probably amino-polysaccharides associated in a protein-amino-sugarlipid complex. The basic character of yolk is probably due to the amino
groups of the amino-sugars since yolk appears to lack protamines and
histones (Monné and Slautterback, 1950).
The data available concerning yolk utilization are all of an indirect
character and are mainly due to the work of Kavanau (1953, 1954). On
the basis of a number of quantitative estimates of free, peptide, and
protein amino-acids he postulated 'four periods of intense yolk proteins
breakdown, one during early cleavage, a second in the late blastula and
extending through hatching, a third just before gastrulation and the
fourth in the late prisma and very early pluteus'. These periods alternate
with periods of protein synthesis. In other words, yolk breakdown would
replenish the amino-acid and peptide pool on which the embryo draws
for its synthesis. However, Kavanau (1958) later found that the method
ALBERTO MONROY AND RACHELE MAGGIO
lasm of the egg and blastomeres. On the other hand there is no need to
emphasize how important it would be to know as much as possible about
the mechanisms and pathways of yolk resorption and of the utilization
of its breakdown products for the synthesis of embryonic proteins.
The appearance of the yolk platelets in the electron microscope
(Afzelius, 1956; our unpublished observations) shows that their main
body consists of tightly-packed tiny electron-dense granules. Each
platelet is surrounded by a thin granular membrane which is often lost
in the homogenates prepared with the usual sucrose procedure. Even
when this happens, the body of the platelet retains its appearance,
i.e., the elementary particles are not dispersed (unpublished observations). In some preparations of blastulae, the main body of some of
the platelets appeared to have patches of dense and clear zones.
Furthermore they were surrounded by a multi-layered membrane
similar to the one described by Karasaki (1959) in the tail-bud
stage of the amphibian embryo and interpreted by this author as an
indication of the decomposition of the main body that begins at its
surface. According to some preliminary observations (unpublished), the
platelets in this condition become lighter as they are sedimented at
higher centrifugal force. This may indeed be an expression of the progress
of their utilization. The significance of the lamellar structures surrounding the yolk platelets is at present obscure. No doubt it suggests the
possible transformation of the platelet into some other cytoplasmic
structure. Indeed, the possibility of the yolk being a source of new
mitochondria has been repeatedly suggested (Bellairs, 1958; Lanzavecchia and La Coultre, 1958; Sung, 1961) but no definite proof is yet
available. Histochemical tests have failed to indicate any change in the
yolk from the uncleaved egg to the pluteus. The general conclusion from
these observations has been that the main components of yolk are
probably amino-polysaccharides associated in a protein-amino-sugarlipid complex. The basic character of yolk is probably due to the amino
groups of the amino-sugars since yolk appears to lack protamines and
histones (Monné and Slautterback, 1950).
The data available concerning yolk utilization are all of an indirect
character and are mainly due to the work of Kavanau (1953, 1954). On
the basis of a number of quantitative estimates of free, peptide, and
protein amino-acids he postulated 'four periods of intense yolk proteins
breakdown, one during early cleavage, a second in the late blastula and
extending through hatching, a third just before gastrulation and the
fourth in the late prisma and very early pluteus'. These periods alternate
with periods of protein synthesis. In other words, yolk breakdown would
replenish the amino-acid and peptide pool on which the embryo draws
for its synthesis. However, Kavanau (1958) later found that the method